1. Field of the Invention
The field of the present invention is that of turbomachines and, more particularly, that of the calibration of the thrust exerted on the bearings of these turbomachines.
2. Description of the Related Art
A turbomachine is made up of numerous parts having, for the most part, transverse extensions in relation to its rotational axis, on which transverse extensions are exerted axial forces generated by the pressures of the air in which they bathe. The existing multiple cavities of pressurized air thus create stresses upon the turning parts (rotors) and upon the fixed parts (stators), which stresses add to the aerodynamic stresses exerted on the blades. Whereas the stresses incurred by the stators are sustained by the different casings, those experienced by the rotors add to these and their resultant is absorbed by the thrust bearing which supports the corresponding rotation shaft. It is necessary to control, for all loads, the level of stresses applied to the bearing, in order to ensure that these stresses are neither too strong, in which case there would be a risk of damage to the bearing, nor too weak, in which case there would be a risk of support losses and of possible vibratory phenomena in the axial direction. In general terms, for reasons of controlling the axial clearance of the compressor, it is arranged that the resulting thrust exerted on the shaft shall be directed forwards.
In the case of a turbomachine comprising a centrifugal compressor, the downstream face of the compressor constitutes a disc extending transversely over the whole of the section of the engine and it can be the focus of considerable stresses, as a function of the pressure exerted thereon. The pressure exerted in the cavity situated downstream of this face constitutes a fundamental element for controlling the thrust exerted on the thrust bearing; it is important, however, that it is controlled perfectly. Examples of such turbomachines are given by the patent applications filed by the Applicant and published under references EP 1881181 and EP 1882825.
As can be seen in FIG. 1, where it is represented in dotted lines, a portion of the air flow emanating from the centrifugal compressor is drawn off in order to feed or pressurize cavities associated with the turbine of the engine. This air makes its way in the first place towards the shaft of the engine, skirting the downstream face of the centrifugal compressor, before taking a longitudinal direction, and its pressure determines the resulting forwardly directed stress which has to be managed. Owing to the rotation of the impeller of the centrifugal compressor, this air is imparted with a rotational movement and its velocity comprises a not insignificant tangential component, by comparison with its centripetal component. This tangential velocity is traditionally expressed by a coefficient Kd, the so-called drag coefficient, which is equal to the ratio between the tangential velocity of the drawn-off air and the linear velocity of the impeller at the considered point. This coefficient therefore has a value which is variable as a function of the radial distance at the considered point, and this generally decreases from the outside towards the central portion of the downstream face of the impeller, owing to a natural correction of the flow towards a more centripetal direction.
The presence of a tangential component for the velocity of the fluid, which is of no practical interest, has the drawback of reducing its static pressure and thus lessening the force applied to the downstream face of the centrifugal compressor. The consequence of this is an axial thrust resultant oriented potentially rearwards, or at least oriented upstream but with insufficient support, which is contrary to the aspired aim to comply with the dimensioning specifications of the turbomachine.